The connection at the root is going to bear 1/2 of the total force any way you figure it. So lets say worst case at a 50 g load for a 5 LB model you will be dealing with a load of 125 lbs. total or 62.5 lbs. at the root. This will be a combination of compressive force towards the center and shear force up and down so your cabin will have to be able to withstand both the compressive and shear force as far as the wing attachment I would make the root of the wing itself out of 1" thick balsa stock into which you set two or three sockets for the rods that you have attached to a 1/32 thick piece aluminum sheet stock which is fastened to the cabin side. I would look at combining this with the window and doorframe so you can get it down the side of the fuse for maximum shear strength. You can fiberglass the into the top sheathing and fuse sides for maximum bonding and strength. I would also fiberglass the cabin ceiling along with the top of the wing sheathing for maximum strength. A hard point to mount your spar on both the wing and fuselage and your done.

Now keep in mind that the actual amount of stress depends upon your gross weight and the 5lb ref. is only an example.